The present invention relates to a circuit arrangement for detecting measured values of a pressure-sensitive resistor mat located in a vehicle seat for detecting seat occupancy, where a first resistor is connected in parallel to the resistor mat, and a voltage divider is provided at which a measured voltage dependent on the resistance of the resistor mat can be tapped.
A circuit arrangement of this type is known from German Patent Application No. 197 27 193. A pressure-sensitive resistor mat in a vehicle seat can be used to determine the weight of the person, or a child seat or another object occupying the vehicle seat. Based on this information, a control unit makes a determination for restraining means (such as airbags) as to whether, based on the vehicle seat occupancy, the airbag should even be triggered in the event of a vehicle crash, or how forcefully it should be expanded to protect the person occupying the vehicle seat.
FIG. 3 shows a circuit arrangement designed according to the cited related art for detecting the measured values of a pressure-sensitive resistor mat. In this circuit, resistor RM represents the pressure-dependent, variable resistance of the resistor mat. A series circuit including a resistor R and a diode D is connected in parallel to resistor RM of the resistor mat. A first transistor T1 with a voltage divider R11, R21 set to a supply voltage of +5V is connected to one output of this parallel circuit, and a second transistor T22 with a voltage divider R12, R22 set to a supply voltage of +5V is connected to the other output of the parallel circuit.
To subsequently test the resistor mat and determine whether a faulty interruption has occurred, first transistor T1 is activated at its input E1, where a current flows exclusively through resistor mat RM, due to the diode polarity, and resulting measured voltage UM1 is tapped at voltage divider R11, R22. Measured voltage UM1 undergoes an analog/digital conversion and is then evaluated by a control unit. If instantaneous pressure-dependent resistance RM of the resistor mat is to be measured, second transistor T2 is activated at its input E2. In this case, diode D is polarized in the forward direction, and a current flows across both branches of parallel circuit RM, R, D. A measured voltage UM2, which undergoes an analog/digital conversion and is then evaluated in the control unit, is tapped at voltage divider R11, R21. For the evaluation, measured voltage UM2 undergoes a threshold value decision determining whether or not the vehicle seat is occupied.
In the case of the described circuit arrangement, a very low current (approximately 80 xcexcA) flows through the resistor mat to determine measured voltage UM2. This very low current can lead to significant measurement errors if low shunts occur in the line conducting the measured signal. A shunt of 100 kW, for example, causes measured voltage UM2 to deviate from the actual measured value by 33%. The circuit arrangement described is relatively expensive, since it requires two transistors with two voltage dividers.
The object of the present invention is therefore to provide a circuit arrangement which responds to faulty shunts on the measured signal line with minimal measured signal deviations and can operate with a relatively small number of components.
The object mentioned above having the features of claim 1, is achieved in that a second resistor is connected in series to the parallel circuit including the resistor mat and the first resistor; a controllable voltage regulator, to which a control voltage dependent on the resistance of the resistor mat is supplied, is connected in parallel to the series circuit; and the output voltage of the voltage regulator is present at a voltage divider. This circuit arrangement, which requires only a few resistors and one voltage regulator, acts like an impedance transformer used to increase the current flowing through the resistor mat for signal evaluation, thereby making the measured signal insensitive to shunts on the measured signal line.
Advantageous embodiments of the present invention are described in the subclaims. The control voltage for the voltage regulator is thus tapped between the second resistor and the parallel circuit including the resistor mat and the first resistor. A temperature-compensated voltage regulator is advantageously used.